In the strict sense of the word, the term "dealumination" refers to the removal of aluminum from zeolite frameworks generally resulting in lattice deficiencies. Nevertheless, in its general use and as it will be used herein it relates to the more complex process which includes the incorporation of other atoms into transient framework vacancies temporarily left by the release of aluminum, such as the isomorphic substitution by silicon into the framework of zeolites.
Zeolitic materials have been demonstrated to have catalytic, adsorption and ion-exchange properties offering a wide field of applications in important industrial processes. Besides the framework topology, the framework Si/Al atomic ratio of zeolites is an important parameter exerting strong influence on properties, such as thermal and hydrothermal stability, hydrophobicity and catalytic activity and selectivity. Generally, an increase of the Si/Al atomic ratio is desirable. However, the framework Si/Al atomic ratio of zeolites prepared by direct synthesis, such as by hydrothermal crystallization, is generally restricted to more or less narrow limits. For example, one of the technically most important members of the zeolite family, faujasite, cannot be directly synthesized with Si/Al atomic ratios substantially higher than 2.5. It is, therefore, of great importance to develop methods for the increase of the Si/Al atomic ratio by chemical modification of zeolite frameworks, such as by the dealumination of zeolites both natural and synthetic.
Processes which increase the silicon-to-aluminum atomic ratio of zeolite structures can be subdivided into the following categories:
a) those involving only the removal of aluminum from the zeolitic framework, thereby resulting in framework vacancies left by the release of aluminum atoms;
b) those including, in addition to the mere extraction of framework aluminum, a second process step in which framework vacancies temporarily left by aluminum release are filled by migrating silicon and oxygen atoms; and
c) those representing true substitution reactions between the aluminum component of the framework and the dealumination agent, being a compound of the element to be incorporated, such as silicon.
Barrer and Makki, Canad. Journ. Chem., 1964, 42, 1481-1487 described the removal of aluminum from the crystal structure by leaching with strong mineral acids. This method is restricted to acid-resistant zeolites, such as, for example, mordenite and clinoptilolite.
In U.S. Pat. No. 3,937,791, a method is described comprising the removal of framework aluminum by treating the zeolite to be dealuminated at 50.degree.-100.degree. C. with an aqueous solution of a chromium salt of a mineral acid whereby the pH is less than 3.5.
In U.S. Pat. Nos. 3,442,795 and 4,093,560, aluminum is removed from the framework of the crystalline aluminosilicate by solvolysis, e.g. hydrolysis, and extracted by the use of complexing or chealating agents.
In U.S. Pat. No. 3,640,681, framework aluminum is removed from zeolites by treatment with acetylacetone.
Aluminum has been removed from the framework of zeolites in the form of volatile AlCl.sub.3 after reacting the zeolite at elevated temperatures with gaseous chlorine compounds. The application of Cl.sub.2 and HCl is described in Ger. Offen. No. 2,510,740, that of phosgene and mixtures of CO and Cl.sub.2 was reported by P. Fejes et al. in React. Kinet. Catal. Lett. 1980, 14, 481-488.
In all these processes, lattice vacancies are left behind after the removal of aluminum from the framework of zeolites.
U.S. Pat. Nos. 3,293,192, 3,402,996 and 3,449,070 describe methods of partial dealumination of the ammonium form of zeolites by heat treatment in presence of water or steam. Under the hydrothermal conditions applied, aluminum leaves the framework and is deposited in the form of oxidic aluminum species in the intracrystalline and/or intercrystalline space while framework vacancies are simultaneously filled up by lattice silicon atoms becoming mobile to some extent under said conditions. This process is, at any rate, accompanied by destruction of the lattice in local regions of the zeolite crystallites and, hence, associated with the formation of a mesopore system. Combined with acid extraction of the deposited aluminum oxide species, the steaming of ammonium exchanged zeolites leads to nearly pure SiO.sub.2 varieties of the respective zeolites containing zeolitic micropores as well as mesopores.
H. K. Beyer and I. Belenykaja described in Stud. Surf. Sci. Catal., 1980, 5, 203-209 a method for direct substitution of aluminum by silicon in zeolite frameworks. This process comprises contacting dehydrated sodium forms of large-pore zeolites at elevated temperatures with gaseous silicon tetrachloride or silicochloroform. Later, this method was improved starting from Y zeolites partially ion-exchanged with Li or by adding LiCl to Na-Y zeolite (Sulikowski et al., J. Phys. Chem., 1989, 93 3240-3243). The tetrachloro-aluminum complex salt formed as reaction product must be removed by washing with water. This dealumination procedure proved to be a reliable process to prepare the pure SiO.sub.2 variety of faujasite. However, it is not possible to prepare partially dealuminated Y zeolites in such a way without formulation of lattice deficiencies because the acidity created during the washing process by hydrolysis of the complex salt affects the aluminum retained in the framework.
In U.S. Pat. No. 4,569,833 framework aluminum is claimed to be substituted by silicon by contacting zeolites at a temperature from ambient to about 200 .degree. C. with gaseous silicon tetrafluoride. Cationic aluminum-fluorine species formed as reaction product were removed by subsequent ion exchange.
In U.S. Pat. No. 4,273,753 there is a process for the dealumination of zeolites comprising contacting the dehydrated hydrogen form of zeolites at elevated temperatures with an inorganic halide or oxyhalide, such at SiCl.sub.4, PCl.sub.3, TICl.sub.4 and CrO.sub.2 Cl.sub.2. The non-halogen component of these compounds are capable of substitution for aluminum in the zeolitic framework leading to incorporation of other elements.
In U.S. Pat. No. 4,503,023 and J. Catal., 1990, 126, 532-545 (Chauvin, et al) a process is described for the dealumination of zeolites and the simultaneous incorporation of silicon into the framework sites originally occupied by aluminum. The process comprises treating an aqueous slurry of the zeolite with a highly diluted aqueous solution of (NH.sub.4).sub.2 [SiF.sub.6 ]. The dealumination degree achievable via this route is limited to a Si/Al ratio of about 7 in the case of Y zeolite. This process requires handling and disposal of large amounts of salt slurries. Further, because of acidity created by hydrolysis of the hexafluoro-silicate complex, undesired acid leaching of framework aluminum occurs to some extent. In the case of zeolites not resistant to acids, the slurry has to be buffered to avoid lattice destruction. Finally, the fluoroaluminate complex formed as reaction product has to be thoroughly washed in order to avoid structure damages during subsequent heat activation procedures.
None of these references disclose a process for substitution of framework aluminum by silicon in zeolites according to the present invention which comprises solid-state reaction between the zeolite to be dealuminated and the crystalline dealuminizing agent.
J. A. Rabo et al. reported in J. W. Hightower (Editor), Proc. 5th Int. Congress Catal., North-Holland Publishing Co., New York, 1973, pp. 1353-1361 the phenomenon of solid-state ion exchange in zeolites. In the last several years, systematic studies were undertaken to comprehend this type of solid-state reaction proceeding at elevated temperatures between lattice cations of zeolites and metal salts (recently reviewed by H. G. Karge and K. H. Beyer, in Stud. Surf. Sci. Catal. 1991, 69, 43-64). In certain systems "contact-induced" ion exchange mediated by intrazeolitic water proceeds even at ambient temperature or slightly higher provided the salt applied as reactant is soluble in water.
In many respects, solid-state ion exchange is superior to the conventional procedure. However, previous solid-state reactions were known to be an adequate method only to modify lattice cation compositions of zeolites while the present invention concerns the application of this type of reaction to chemical modification of zeolitic frameworks.